The present invention relates to shift controls of an automatic transmission for a vehicle and, more particularly, to shift control system and method for an automatic transmission, which can interchange automatic and manual shifts.
As is well known in the art, the automatic transmission is constructed to switch the transmission paths of a drive power through a gear train composed of planetary gear sets and so on, by means of engagement devices such as clutches and brakes, thereby to set a plurality of gear stages. Moreover, a shift to each of the gear stages by actuating shift valves (or change-over valves) to feed or release the oil pressure to predetermined friction devices in response to a first signal such as a throttle opening relating to an engine load and a second signal relating a vehicle speed. In other words, the substantial shifts are performed by switching the shift valves. If, therefore, these shift valves were manually switched, the manual shifts could be accomplished. One example of the systems capable of interchanging the automatic shift mode and the manual shift mode has been proposed in Japanese Patent Publication No. 48-210.
This proposal is constructed such that the automatic shift mode or the manual shift mode can be selected by the shift lever. In case the shift lever is in a first position to select the automatic shift mode, the shift is effected on the basis of a signal indicating the running state such as the throttle opening and the vehicle speed. In case, on the other hand, the shift lever is in a second position to select the manual shift mode, one of the 1st to 3rd speeds of the gear stages to be selected in the automatic shift mode is selected by the shift lever.
As described above, the automatic shifts are effected on the basis of the running state of the vehicle and are frequently achieved irrespective of the intention of the driver. If, therefore, the shifts are not smooth, the driver will be given a discomfortable feel. In the aforementioned system of the prior art, therefore, the individual gear stages are ordinarily set in the existing system, too, such that the shifts may not excessively occur in the automatic shift mode. Since, on the contrary, the manual shift is based upon the intention of the driver, the shift timing and the running state may often fail to match. In order to reduce the shift shocks at the mismatch, it is preferable to reduce the difference between the gear ratios and accordingly the difference in the torque before and after the shift. In the existing system, however, the gear state to be selected in the automatic transmission mode is also selected in the manual shift mode so that the torque difference may possibly be increased across the shift to deteriorate the shift shocks. If, on the contrary, the difference between the gear ratios is reduced to lighten the shift shocks at the manual shifting time, the shift is accomplished between the gear stages of small gear ratios, too, in the automatic shift mode. Then, the shifts are caused frequently enough to invite the so-called "busy shifts" so that the passengers may possibly be given the discomfortable feeling.